1. Field of the Invention
This invention relates to a joint boot made of a resin, and more particularly to a bellows-like joint boot made of resin for use in a constant velocity joint for automotive vehicles or the like.
2. Description of the Related Art
A joint of a driving shaft for automotive vehicles or industrial machines is equipped with a joint boot in order to maintain the grease encapsulated therein or to prevent the dirt or the like from entering.
As illustrated in FIG. 9, a typical joint boot 100 is made up of a large-diameter cylinder part 102 to be fitted on a housing part of a constant velocity joint, a small-diameter cylinder part 104 disposed coaxially to be spaced apart from the large-diameter cylinder part 102 and adapted to be fitted to a shaft, and a bellows part 106 interlinking both parts. On outer circumferential surfaces of the large-diameter cylinder part 102 and the small-diameter cylinder part 104, there are provided respectively recesses 108, 110 for fixation for receiving therein respective fastening clamps. The small-diameter cylinder part 104 is, as shown in FIG. 10, fixed on an outer periphery of a shaft 122 with the aid of a ring-form fastening clamp 120 mounted on the recess 110 for fixation. The large-diameter cylinder part 102 is likewise fixed, although not shown, on an outer periphery of the housing part.
The joint boot of this type can be molded by an injection blow molding procedure of a thermoplastic resin. In that case, an extrusion dye 50 and an injection mold 52 are matched as shown in FIG. 8A and a melted resin is injected from the extrusion dye 50 into the cavity of the injection mold 52. Then while moving the injection mold 52 upwardly, a parison 64 is drawn out as shown in FIG. 8B, and a blow mold 54 is closed to conduct a blow molding by admitting gas into the parison 64, as shown in FIG. 8C. By subsequent demolding and cutting out of a bottom section 76 as shown in FIG. 8D, the joint boot made of resin is thus molded.
Where a joint boot is molded by the injection blow molding procedure in this way, it is a conventional practice to set a boundary X0 between the injection molding section and the blow molding section, as shown in FIG. 10, to be within the recess 110 for fixation, which is defined at the outer periphery of the small-diameter cylinder part 104, more particularly on a wall surface 110a on the bellows part side of the recess 110. The setting of the boundary X0 within the recess 110 for fixation however runs into the problem that, as indicated in FIG. 11, a burr 124 is likely to be produced at a shoulder portion 110b on the bellows part side of the recess 110 for fixation.
According to the present inventors, the production of the burr 124 has been turned out to be ascribed to the mechanism that will be described below: That is, when the extruder die and the molding product are separated from each other after blow molding, as shown in FIG. 12A, a melted resin 136 in a drawn state clings to a taper surface 134 at a top of a core 132 of the extruder die 130. When it proceeds to the next molding step in the state of the residual resin 136 clinging like this, in matching an injection mold 140 to the extruder die 130 as shown in FIG. 12B, the residual resin 136 catches on the outer peripheral edge of an opening of a cavity 142, thereby being extended radially outwardly and ultimately pinched between the extruder die 130 and the injection mold 140. As a consequence, the residual resin 136 remains on the molded product, as a result of which the burr is produced at the boundary between the injection molding section and the blow molding section.
On the other hand, a problem with the conventional resin joint boot of this kind is that since above all, the small-diameter cylinder part causes cracking at its recess for fixation when rotated in the state of a bending deformation at a wide angle, the joint boot is poor in durability. This is due to the fact that in particular, on the small-diameter cylinder part side, a first crest portion 112 of the bellows part 106 adjacent to the small-diameter cylinder part 104 is subjected to recurring deformation in the directions as indicated in an arrow Y0 when rotated under bending deformation at a wide angle, as shown in FIG. 10 and consequently, a stress due to it acts on an extremity 110c of the recess 110 for fixation on the bellows part 106 side.